Gary Underhill, age 9, of St. John, New Brunswick, Canada, for his question:

Is there really a rock that floats?

Yes there is, and some people let it float in the bathtub. It is a pasty grey rock called pumice. Its surface is rough but not harsh. Pumice is fine for rubbing away those patches of dead scruffy skin that form on the heels and other parts of the feet. It feels rough because the outside is bumpy with little pits and pockets. The inside is a foamy mixture of rock and tiny bubbles filled with air. This is the secret that makes pumice float.

Ordinary rocks sink because they are solid and heavier than water. Pumice floats because its airy bubbles make it lighter than water. It got those bubbles because it started out as hot, foamy lava flowing from a fiery volcano. The runny lava cooled fast in the chilly air. Its bubbles of gas did not have time to escape. So when the pumice set hard, they were sealed inside.